1. Field of the Invention
The present invention relates to a duplex printing apparatus and duplex printing method that print a plurality of images on each of front and back surfaces of a continuous sheet.
2. Description of the Related Art
There has been provided a printing apparatus that sequentially prints a plurality of images on a continuous sheet, and cuts the sheet on an image (page) basis. In such a printing apparatus, a conveyance state of the continuous sheet is influenced by an individual difference of the printing apparatus, the type or width of the sheet, use environment, or the like, and therefore a conveyance distance of the continuous sheet per unit time includes a certain level of error. For this reason, in order to prevent the continuous sheet from being cut in the middle of an image, a method that, between images, provides a non-image area printed with a cut mark indicating a cutting position, and on the basis of timing at which a detector detects the cut mark, cuts the continuous sheet with a cutter is used.
However, in the case where in order to detect such a cut mark, the detector performs a reading operation throughout the continuous sheet, a pattern in an image, which is similar to the cut mark, may also be determined as the cut mark to cut the continuous sheet on the middle of the image.
To address such a problem, Japanese Patent Laid-Open No. 2012-158122 discloses a method that performs a reading operation by a detector only on a non-image area printed with a cut mark. Also, Japanese Patent Laid-Open No. 2012-158122 discloses a configuration in which by focusing on the fact that an error in conveyance amount of a continuous sheet is increased along with an increase in conveyance distance of the continuous sheet, i.e., an increase in size of an image precedently printed, a size of a non-image area between two successive images is adjusted depending on a size of a precedent image. According to such a Japanese Patent Laid-Open No. 2012-158122, in a non-image area having a size enough to include a conveyance error of the continuous sheet, a cut mark can be detected, and therefore even in the case where a conveyance error occurs, the continuous sheet can be cut in a correct position.
However, with the configuration of Japanese Patent Laid-Open No. 2012-158122, a function of duplex printing that prints images on front and back surfaces of a continuous sheet may not be fulfilled. In the following, this is specifically described.
In general, even in the case of printing a plurality of images on front and back surfaces, a cutting position is common to the front and back surfaces. Accordingly, in such a printing apparatus, a layout of a plurality of images is determined such that one image to be printed on a front surface and one image to be printed on a back surface just on the back side of the front surface are coincident with each other to the utmost extent in terms of size and position in a conveyance direction. For this reason, one non-image area on a front surface and one non-image area on a back surface just on the backside of the front surface are also substantially coincident with each other in terms of size.
On the other hand, in such a duplex printing apparatus, it is typical to place a reversing roller downstream of a printing unit that prints an image, reverse here a continuous sheet in terms of front and back surface, which has been subjected to front surface printing, and then send the continuous sheet to the printing unit again. In doing so, the continuous sheet sent from the reversing roller is reversed in terms of the front and back surfaces thereof, as well as in terms of fore and rear ends thereof in a conveyance direction. That is, a part serving as a rearmost end at the time of printing the front surface serves as a foremost end at the time of printing the back surface. As in Japanese Patent Laid-Open No. 2012-158122, in the case of adjusting a size of an individual non-image area depending on a size of a precedent image at the time of printing a front surface, a size of a non-image area at the time of printing a back surface is adjusted depending not on a size of a precedent image but on a size of a subsequent image.
FIG. 10 is a schematic diagram for specifically describing such a situation. For example, consider the case where on a front surface, an image A, image B, and image C are sequentially printed, and on back surfaces of the images A, B, and C, images D, E, and F respectively having the same sizes as those of the images A, B, and C are printed. In this case, according to Japanese Patent Laid-Open No. 2012-158122, at the time of surface printing that prints the images while conveying a continuous sheet in an A-direction, a size WA of a non-image area 1 between the images A and B is set depending on a size LA of the image A. Also, a size WB of a non-image area 2 between the images B and C is set depending on the size LB of the image B, and a size WC of a non-image area 3 subsequent to the image C is set depending on the size LC of the image C.
On the other hand, at the time of back surface printing, the continuous sheet is reversed in terms of fore and rear ends thereof, and therefore the conveyance direction is switched to a B direction. Accordingly, the images F, E, and D are printed in this order so as to respectively coincide in position with the images C, B, and A. As a result, on a back surface side, the continuous sheet is conveyed and printed in the order of non-image area 3→image F→non-image area 2→image E→non-image area 1→image D, and detection of cut marks and cutting are also performed in this state.
According to an original purpose of Japanese Patent Laid-Open No. 2012-158122, an error in conveyance amount of a continuous sheet is increased/decreased depending on a size of an image precedently conveyed, and therefore the size of the non-image area 1 at the time of a cutting operation should be adjusted depending on the size of the image E to be conveyed precedently to the non-image area 1. However, in the case of duplex printing employing a technique in Japanese Patent Laid-Open No. 2012-158122 as described above, at the time of the back surface printing where a cutting process is actually performed, the size of the non-image area 1 is adjusted depending not on the size of the image E to be precedently conveyed but on the size of the image A having substantially the same size as that of the image D to be subsequently conveyed. That is, a non-image area appropriate to an error in conveyance amount is not provided, and a problem to be solved by Japanese Patent Laid-Open No. 2012-158122 cannot be solved.